Available Space Misreported by USB drive?

Dennis Faas's picture

Infopackets Reader Jeff R. writes:

"I recently bought a 512MB USB 'TravelDrive' made by Memorex. Everything works great, except I can't figure out why the drive is only showing 480MB of usable, available space. In correspondence with a Memorex tech, I asked why this was so, and his answer was rather vauge and confusing. Does it really take 32 MB of overhead to format a USB drive? If this is the case, it's probably a good thing I didn't buy a 128MB drive!"

Doug's Response:

It's questions like these that make me feel glad I've been involved in the computer industry for as long as I have. ;-)

In fact, keeping track of all the bits, bytes, and where they are used has always interested me. I still smile when I remember how confused I was when I discovered that 1K was really 1024!

Getting back to your question: The Flash drive really does have 512 MB of space. The brief techie explanation is this:

The file system ["reference table"] is what's taking up most of the room. The rest of the space is being eaten up by dividing the TravelDrive into sectors (just as with a hard drive or floppy disk, etc).

In short: the more sectors, the bigger the table. That table is used by your Operating System to locate a file for you. It also can determine when the file was written, last read, last modified, if the file is writable or is read-only and lots of other needed info to keep things running smoothly.

Also worth noting: the file system table makes it possible for you to enter a file name into your word processor and end up with the entire file loaded and ready to go. If you did not have that table, you would have to know all of the 'absolute addresses' (and their order) of the file on the drive to retrieve it. You think computers are confusing now just imagine if you had to remember a long string of numbers and letters that made no sense at all to get the file you wanted. ;-)

So, just consider the 'lost' space as "operational overhead."

You really are getting the full byte count listed for the size on the device, though. However, it is RAW space. As soon as a file system type (FAT32, NTFS, etc.) is defined and the drive is prepared for use, what you are left with is the remaining available free space.

That used space is not directly accessible through normal system access methods so it isn't shown. If it were, folks would get into it and either erase it or modify it. Both actions would render the drive unusable (a reformat of the drive will make it usable again) and any data on the drive would be lost. You'll agree, I'm sure, that it is better to seemingly lose a block of drive space than to run the risk of losing a drive,

An interesting note: FAT16, FAT32, and NTFS use entirely different filesystem table formats. This is why (for example) NTFS drives aren't compatible with FAT32-based operating systems (such as Windows 98).

Have fun!

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